Formatted messages can be sent in 2 ways.
Using Compose mode, you can use bold or italicized font plus bullet points and code snippets to format your message. Refer to the Knowledge Article found here for further information on expanding your writing area with Compose mode.
Alternatively, you can start typing your message in a chat window, then highlight it with the mouse and the formatting options will automatically appear (Figure 1):
Figure 1 Formatting options